Method and apparatus for measurement and control of photomask to substrate alignment

ABSTRACT

A method, structure, system of aligning a substrate to a photomask. The method includes: directing incident light through a pattern of clear regions transparent to the incident light in an opaque-to-the-incident-light region of a photomask, through a lens and onto a photodiode formed in a substrate, the photodiodes electrically connected to a light emitting diode formed in the substrate, the light emitting diode emitting light of different wavelength than a wavelength of the incident lights; measuring an intensity of emitted light from light emitting diode; and adjusting alignment of the photomask to the substrate based on the measured intensity of emitted light.

The present application is a division of U.S. patent application Ser.No. 12/026,763 filed on Feb. 6, 2008.

FIELD OF THE INVENTION

The present invention relates to the field of optical photolithography;more specifically, it relates to a structure and method for determiningand adjusting photomask and lens to wafer alignment in an opticalphotolithography system.

BACKGROUND OF THE INVENTION

Current optical photolithographic techniques are unable to use lightwith a wavelength below 193 nm because fused silica (silicon dioxide) ofconventional mask substrates is opaque to wavelengths below 193 nm.Substrate materials that are transparent to light with a wavelengthbelow 193 nm have high thermal coefficients of expansion compared tosilicon dioxide and thus expand and contract far too much to be usedreliably in sub-193 nm lithography. While some schemes have beenproposed to overcome this problem for those fabrication levels commonlyreferred to as front-end-of-line (FEOL) which are substrate level, thereare no schemes for overcome this problem for those fabrication levelscommonly known as back-end-of-line (BEOL) fabrication levels which areinterconnection/wiring levels. Because the minimum feature sizeprintable in an optical photolithography system is a function of thewavelength of the actinic radiation (shorter wavelengths allowingsmaller feature sizes) it would be useful to the industry to overcomethe deficiencies and limitations described hereinabove.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a method; comprising:directing incident light through a pattern of clear regions transparentto the incident light in an opaque-to-the-incident-light region of aphotomask, through a lens and onto a photodiode formed in a substrate,the photodiode electrically connected to a light emitting diode formedin the substrate, the light emitting diode emitting light of differentwavelength than a wavelength of the incident light; measuring anintensity of emitted light from the light emitting diode; and adjustingalignment of the photomask to the substrate based on the measuredintensity of emitted light.

A second aspect of the present invention is a structure, comprising: oneor more alignment monitors formed in a substrate and arranged in a rowin a first direction, each alignment monitor of the one or morealignment monitors comprising a respective photodiode electricallyconnected to a respective light emitting diode, each respective lightemitting diode configured to emit a different wavelength of light, eachrespective photodiode comprising first regions of the substrate thatemit electrons when struck by incident light interdigitated with secondregions of the substrate that do not emit electrons when struck by theincident light.

A third aspect of the present invention is an apparatus for aligning asemiconductor substrate to a photomask, the substrate including an arrayof light emitting diodes, each light emitting diode of the array oflight emitting diodes configured to emit light in a different range ofwavelengths, comprising: an X-Y-θ stage configured to hold thesemiconductor substrate; a light source; a lens; a mask holderconfigured to hold the photomask between the light source and lens; aslit between the mask holder and the lens; means for aligning alignmenttargets on the substrate to alignment marks on the photomask; means fordirecting incident light onto the substrate; means for measuringintensities of light, emitted from the array of light emitting diodes,in different wavelength ranges; and a sub-system configured to directtemperature controlled gas (i) over the photomask based on signalsreceived from the means for measuring intensities of light (ii), overthe lens based on the signals received from the means for measuringintensities of light, or (iii) over both the photomask and the lensbased on the signals received from the means for measuring intensitiesof light.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of an illustrative embodiment when readin conjunction with the accompanying drawings, wherein:

FIG. 1 is a top view of an exemplary integrated circuit wafer on whichthe embodiments of the present invention may be practiced;

FIG. 2 is a higher magnification view of the wafer of FIG. 1,illustrating positioning of alignment monitors according to theembodiments of the present invention;

FIG. 3 is a top view of an exemplary alignment structure according toembodiments of the present invention;

FIGS. 4 and 5 are a cross-sectional views illustrating the relationshipbetween alignment structures and corresponding patterns on photomasksaccording to embodiments of the present invention;

FIG. 6. is a cross-sectional view illustrating the relationship betweenalignment structures and corresponding patterns on photomasks accordingto a modified embodiment of the present invention;

FIGS. 7A through 7C are top views illustrating an example of how theembodiments of the present invention can distinguish between degrees ofalignment between a wafer and a corresponding photomasks;

FIG. 8 illustrates the arrangement of components of the six alignmentmonitors illustrated in FIG. 2;

FIGS. 9, 10 and 11 are cross-sectional views of alignment structuresaccording to the present invention formed in integrated circuits duringfabrication of the integrated circuit;

FIG. 12 is schematic diagram of a first optical photolithography systemaccording to embodiments of the present invention;

FIG. 13 is schematic diagram of a second optical photolithography systemaccording to embodiments of the present invention;

FIG. 14 illustrates a first option that may be applied to the first andsecond optical photolithography systems;

FIG. 15 illustrates a second option that may be applied to the first andsecond optical photolithography systems; and

FIG. 16 illustrates that both the first and second option may be appliedto the first and second optical photolithography systems.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a top view of an exemplary integrated circuit wafer on whichthe embodiments of the present invention may be practiced. It is commonpractice to fabricate multiple integrated circuit chips simultaneouslyon thin, disc shaped semiconductor substrates called wafers. Commonwafer diameters are 100 mm, 200 mm and 300 mm with thicknesses in thehundreds of micron range. In one example, wafers consist ofsingle-crystal silicon. In one example, wafers comprise an upper singlecrystal silicon layer separated from a lower single-crystal siliconlayer by a buried oxide (BOX) layer. These latter wafers are also calledsilicon-on-insulator (SOI) wafers. After fabrication of the integratedcircuit chips is complete, the individual chips are separated in anoperation called dicing. In FIG. 1, a wafer 100 includes an array ofintegrated circuit chips 105 separated by horizontal (X-direction) kerfs110A and vertical (Y-direction) kerfs 110B. Wafer 100 includes a notch115 for orientating the wafer in various fabrication systems. There maybe several notches. In FIG. 1, a line 116 running from notch 115 througha center 117 of wafer 100 defines the vertical or Y-direction. Thehorizontal or X-direction is perpendicular to line 116 and in the sameplane as line 116. In some wafers, notch 115 is offset from line 116.Kerfs 110A and 110B are also called streets.

FIG. 2 is a higher magnification view of the wafer of FIG. 1,illustrating positioning of alignment monitors according to theembodiments of the present invention. In FIG. 2, only a portion of wafer100 is illustrated. In FIG. 2, formed in kerf 110A is a first set ofalignment monitors 120X comprised of a first alignment monitor device120XA, a second alignment monitor device 120XB and a third alignmentmonitor device 120XC. Formed in kerf 110B is a second set of alignmentmonitors 120Y comprised of a fourth alignment monitor device 120YA, afifth alignment monitor device 120YB and a sixth alignment monitordevice 120YC. The intersection of kerfs 110A and 110B is designatedcorner 110C.

First set of alignment monitor sets 120X will detect a degree ofalignment between a photomask and wafer 100 in the X direction. Secondset of alignment monitors 120Y will detect a degree of alignment betweenthe photomask and wafer 100 in the Y-direction as described infra.Alignment monitor sets 120X and 120Y are aids to photolithographicfabrication operations of BEOL levels.

Most modern photolithography systems are step and expose or step andscan systems, in that the photomask used in the system has patterns forless integrated circuit chips than the number that can printed on wafer100. These photomasks are often called reticles. Exemplary reticles maycontain one, two, four or other numbers of chip exposure fields, eachchip exposure field containing a chip 105, one kerf 110A, one kerf 110Band one corner 110C. To expose an entire wafer, the wafer is aligned tothe mask and exposed, and then the wafer is stepped to another position,aligned to the mask and then exposed again. This is repeated as manytimes a required to expose all the integrated circuit chip positions onthe wafer. There need only be one instance of first and second sets ofalignment monitors 120X and 120Y for each region of wafer 100 that isdefined by the reticle.

In an alternative arrangement both first set of alignment monitors 120Xand second set of alignment monitors are contained in either kerf 110Aor 110B or corner 110C. However, first set of alignment monitors 120Xremain aligned in a row in the X direction from first alignment monitordevice 120XA to second alignment monitor device 120XB to third alignmentmonitor device 120XC and second set of alignment monitors 120Y remainaligned in a column in the Y direction from fourth alignment monitordevice 120YA to fifth alignment monitor device 120YB to sixth alignmentmonitor device 120YC.

FIG. 3 is a top view of an exemplary alignment structure according toembodiments of the present invention. In FIG. 3, an alignment monitordevice 120 (which represents each of alignment monitors 120XA, 120XB,120XC, 120YA, 120YB and 120YC of FIG. 2) comprises a light emittingdiode (LED) 125 and a photodiode 130. Photodiode 130 includes P-dopedregions 135 interdigitated with N-doped regions 140. In one example, theperimeter of array 130 abuts dielectric isolation. P-doped regions 135are electrically connected in parallel to a first terminal of LED 125 bywires 132 and N-doped regions 140 are electrically connected in parallelto a second terminal of LED 125 by wires 133. Light striking P-dopedregions 135 of photodiode 130 will generate current flow (i.e.,electrons) to LED 125 causing the LED to emit light. Light strikingN-doped regions 140 of photodiode 130 will not generate current flow toLED 125. It should be noted that no external power source is requiredfor alignment monitors according to the embodiments of the presentinvention to operate.

An exemplary integrated circuit comprises a semiconductor substratecontaining first and second sets of alignment monitors 120X and 120Y andvarious other devices such as field effect transistors (FETs), bipolartransistors, diodes, capacitors, resistors formed in the substrate andinterconnect levels formed in sequence over a top surface of the wafer.In one example, a lowermost (i.e., closest to the wafer) interconnectlevel is formed from polysilicon (often used to form gates of FETs), anext interconnect level includes metal contacts, and subsequentinterconnect levels from a first to a last wiring level include metalwires and metal filled vias for interconnecting the wires in the variouswiring layers. The last wiring level is the uppermost (i.e., farthestfrom the wafer) interconnect level. This structure further describedinfra with reference to FIGS. 9, 10 and 11. In a preferred embodiment,wires 132 and 133 are formed in the lowermost (polysilicon) level. Inanother preferred embodiment wires 132 and 133 are formed in the contactlevel. In still another preferred embodiment wires 132 and 133 areformed in the first wiring level. In order to use the alignment monitorsof the embodiments of the present invention on a maximum number ofinterconnect levels, it is advantageous that the alignment monitors befunctional as early in the fabrication process as possible.

Photodiode 130 has a width W and a length L. In one example L is betweenabout 20 microns and about 100 microns. In one example W is betweenabout 10 microns and about 30 microns. P-doped regions 135 have a width(measured in the L direction) of A and N-doped regions 140 have a widthB (measured in the L direction). In one example A is equal to B. In oneexample A is less than B. In one example A and B are each independentlybetween about 30 nm to about 200 nm.

Exemplary photodiodes and methods of manufacture are described in U.S.Pat. No. 5,252 851 to Mita et al., issued Oct. 12, 1993 and U.S. Pat.No. 5,418, 396 to Mita, issued May 23, 1995 which is hereby incorporatedby reference in their entireties. An exemplary LED and method ofmanufacture is described in United States Patent Publication2001/0007359 to Ogihara et al., published Jul. 12, 2001, which is herebyincorporated by, reference in its entity.

FIGS. 4 and 5 are a cross-sectional views illustrating the relationshipbetween alignment structures and corresponding patterns on photomasksaccording to embodiments of the present invention. In FIG. 4 a photomask145 having opaque regions 150 interdigitated by clear regions 155 isaligned perfectly to corresponding N- doped regions 140 and P-dopedregions 135 of photodiode 130 (see dashed lines). Light passing throughregions 155 strikes P-doped regions 135 but not N-doped regions 140.Thus a maximum amount of current for a fixed light intensity isgenerated. In FIG. 5, photomask 145 is offset by a distance equal toA/2. Half the light passing through regions 155 strikes P-doped regions135 and half the light strikes N-doped regions 140 (see dashed lines).Thus only half the maximum amount of current for the fixed lightintensity is generated.

It is clear that an alignment monitor on a wafer comprising alternatingbands of N-and P-doped regions can be used to measure a degree ofmisalignment between the wafer and photomask, the photomask having apattern of alternating clear and opaque regions arranged to correspondin a predetermined manner to the pattern of P-doped and N-doped regions.This is described in more detail infra in reference to FIGS. 7A, 7B and7C.

FIG. 6. is a cross-sectional view illustrating the relationship betweenalignment structures and corresponding patterns on photomasks accordingto a modified embodiment of the present invention. Since light maydiffract in passing through regions 155 as of the offset approaches A,large degrees of misalignment (greater than A) will not bedistinguishable from very small degrees of misalignment. This effect canbe offset by making A can smaller than B as illustrated in FIG. 6. InFIG. 6, B is about 3 times A and perfect mask to wafer alignment isshown. However, even if photomask 145A is offset by more than A (butless than 3A), the next P-doped region will not be struck by light.

FIGS. 7A through 7C are top views illustrating an example of how theembodiments of the present invention can distinguish between degrees ofalignment between a wafer and a corresponding photomasks. In FIGS. 7A,7B, and 7C, the use of first, second and third alignment monitors 120XA,120XB and 120XC will be described but the description is applicable tofourth, fifth and sixth alignment monitors 120YA, 120YB and 120YC (seeFIG. 2) by translation from the X direction to the Y direction by a 90degree rotation.

In FIG. 7A, the P-doped and N-doped regions (labeled P and Nrespectively and corresponding to P-doped regions 135 and N-dopedregions 140 of FIG. 3) each have widths A measured in the C-direction.Photodiode 130XB is between photodiodes 130XA and 130XC. Photodiode130XA is spaced a distance A from photodiode 130XB and photodiode 130XCis spaced a distance A from photodiode 130XB, where N is a positive oddinteger greater than or equal to 3. In FIG. 7A, first, second and thirdsets 160XA, 160XB and 160XC of clear regions 155 (dashed boxes) in aphotomask are shown. Each clear region 155 has a width A. Adjacent clearregions within each set 160XA, 160XB and 160XC are spaced apart adistance A. Adjacent clear regions 155 from different sets of clearregions are spaced a distance 3 times A/2 apart.

In FIG. 7A, first, second and third sets of clear regions 160XA, 160XBand 160XC and first, second and third photodiodes 130XA, 130XB and 130XCare shown when the photomask and wafer are in perfect alignment. Thisperfect alignment places clear regions 155 of second set of clearregions 160XB over the N-doped regions but not over the P-doped regionsof second photodiode 130XB. Thus no current is generated by photodiode130XB and LED 125XB emits no light (zero intensity). However, clearregions 155 of first and third set of clear regions 160XA and 160C arealigned respectively over half of the N-doped regions half of theP-doped regions of first and third photodiodes 130XA and 130XC. Thushalf the maximum current is generated by photodiodes 130XA and 130XC andLEDs 125XA and 125XC each emit half the maximum amount of light (halfmaximum intensity).

In FIG. 7B, first, second and third sets of clear regions 160XA, 160XBand 160XC and first, second and third photodiodes 130XA, 130XB and 130XCare shown when the photomask is misaligned a distance A/2 to the left(negative X direction). This negative shift in alignment places clearregions 155 of first set of clear regions 160XA over the P-doped regionsbut not over the N-doped regions of first photodiode 130XA. Thus themaximum amount of current is generated by photodiode 130XA and LED 125XAemits the maximum amount of light (maximum intensity). Clear regions 155of second set of clear regions 160XB are placed over half of the N-dopedregions half of the P-doped regions of second photodiode 130XB. Thushalf the maximum current is generated by photodiode 130XB and LED 125XBemits half the maximum amount of light (half maximum intensity). Clearregions 155 of third set of clear regions 160XC are placed over theN-doped regions but not over the P-doped regions of third photodiode130XC. Thus no current is generated by photodiode 130XC and LED 125XCemits no light (zero intensity).

In FIG. 7C, first, second and third sets of clear regions 160XA, 160XBand 160XC and first, second and third photodiodes 130XA, 130XB and 130XCare shown when the photomask is misaligned a distance A/2 to the right(positive X direction). This positive shift in alignment places clearregions 155 of first set of clear regions 160XA over the N-doped regionsbut not over the P-doped regions of first photodiode 130XA. Thus nocurrent is generated by photodiode 130XA and LED 125XA emits no light(zero intensity). Clear regions 155 of second set of clear regions 160XBare placed over half of the N-doped regions half of the P-doped regionsof second photodiode 130XB. Thus half the maximum current is generatedby photodiode 130XB and LED 125XB emits half the maximum amount of light(half maximum intensity). Clear regions 155 of third set of clearregions 160XC are placed over the P-doped regions but not over theN-doped regions of third photodiode 130XC. Thus the maximum amount ofcurrent is generated by photodiode 130XC and LED 125XC emits the maximumamount of light (maximum intensity).

It should be noted that the amount of light generated by first, secondand third LEDs 125XA, 125XB and 125XC depends upon the amount anddirection of misalignment. Table I shows the fraction of maximum lightintensity for a number of exemplary misalignments.

TABLE I MISALIGNMENT LED ONE LED TWO LED THREE −A/2 0 ½ 1 −A/4 ¼ ¼ ¾None ½ 0 ½ +A/4 ¾ ¼ ¼ +A/2 1 ½ 0

It should be understood, that there are very many combinations of Pdoped region width, N-doped region width, photodiode to photodiodedistance and photomask clear region widths, spacings and clear regionset to set spacings that may be used besides the specific widths andspacing illustrated in FIG. 7A. It should also be understood that first,second and third photodiodes need to emit different wavelengths of lightso the intensities from the different LEDs can be measuredindependently.

FIG. 8 illustrates the arrangement of components of the six alignmentmonitors illustrated in FIG. 2. In FIG. 8, first set of alignmentmonitors 120X comprise first alignment monitor device 120XA, secondalignment monitor device 120XB and a third alignment monitor device120XC. Second set of alignment monitors 120Y comprises a fourthalignment monitor device 120YA, a fifth alignment monitor device 120YBand a sixth alignment monitor device 120YC. First alignment monitordevice 120XA comprises first LED 125XA and first photodiode 130XA andthe first photodiode includes interdigitated P-doped regions 135 andN-doped regions 140. Second alignment monitor device 120XB comprisessecond LED 125XB and second photodiode 130XB and the second photodiodeincludes interdigitated P-doped regions 135 and N-doped regions 140.Third alignment monitor device 120XC comprises third LED 125XC and thirdphotodiode 130XC and the third photodiode includes interdigitatedP-doped regions 135 and N-doped regions 140. Fourth alignment monitordevice 120YA comprises a fourth LED 125YA and fourth photodiode 130YAand the fourth photodiode includes interdigitated P-doped regions 135and N-doped regions 140. Fifth alignment monitor device 120YB comprisesa fifth LED 125YA and a fifth photodiode 130YA and the fifth photodiodeincludes interdigitated P-doped regions 135 and N-doped regions 140.Sixth alignment monitor device 120YC comprises a sixth LED 125YC andsixth photodiode 130YC and the sixth photodiode includes interdigitatedP-doped regions 135 and N-doped regions 140.

First, second and third photodiodes 130XA, 130XB and 130XC are arrangedin a row in the X direction with the second photodiode between the firstand third photodiodes. P-doped regions 135 and N-doped regions 140 offirst, second and third photodiodes 130XA, 130XB, 130XC havelongitudinal axes parallel to each other in the Y direction. Fourth,fifth and sixth photodiodes 130YA, 130YB and 130YC are arranged in acolumn in the Y direction with the fifth photodiode between the fourthand sixth photodiodes. P-doped regions 135 and N-doped regions 140 offourth, fifth and sixth photodiodes 130YA, 130YB and 130YC longitudinalaxes parallel to each other in the Y direction.

Each of first, second, third, fourth, fifth and sixth LEDs 125XA, 125XB,125XC, 125YA, 125YB and 125YC emit light of a different wavelength, sosix different LEDs types are required, while only one photodiode type isrequired. The sixth wavelengths emitted by the first, second, third,fourth, fifth and sixth LEDs 125XA, 125XB, 125XC, 125YA, 125YB and 125YCare different from the wavelength of light used to activate first,second, third, fourth, fifth and sixth photodiodes 130XA, 130XB, 130XC,130YA, 130YB and 130YC. The wavelength of light used to activate first,second, third, fourth, fifth and sixth photodiodes 130XA, 130XB, 130XC,130YA, 130YB and 130YC may be the same as the wavelength used to exposea photoresist layer formed on the wafer containing first and second setsof alignment monitors 120X and 120Y or a wavelength that the photoresistlayer is not sensitive to. In one example, the wavelength of light usedto expose the photoresist layer is less than or equal to 193 nm. In oneexample, the wavelength of light used to expose the photoresist layer is157 nm. In one example, first, second, third, fourth, fifth and sixthLEDs 125XA, 125XB, 125XC, 125YA, 125YB and 125YC emit light in the rangeof about 430 nm to about 940 nm. The emission wavelength of LEDs can becontrolled by varying the forward bias voltage (which may require avoltage adjustment circuit between the photodiode and the LED in theembodiments of the present invention) and/or by selection of the LED dyematerial (and concentration) as illustrated in TABLE II.

TABLE II Wavelength Voltage (nm) (V) Dye Material 940 1.5 GaAlAs/GaAs880 1.7 GaAlAs/GaAs 850 1.7 GaAlAs/GaAs 660 1.8 GaAlAs/GaAs 635 2.0GaAsP/GaAs 633 2.2 InGaAlP 620 2.2 InGaAlP 612 2.2 InGaAlP 605 2.1GaAsP/GaP 595 2.2 InGaAlP 592 2.1 InGaAlP 585 2.1 GaAsP/GaP 574 2.4InGaAlP 570 2.0 InGaAlP 565 2.1 GaP/GaP 560 2.1 InGaAlP 555 2.1 GaP/GaP525 3.5 SiC/GaN 505 3.5 SiC/GaN 470 3.6 SiC/GaN 430 3.8 SiC/GaN

FIGS. 9, 10 and 11 are cross-sectional views of alignment structuresaccording to the present invention formed in integrated circuits duringfabrication of the integrated circuit. In FIGS. 9, 10 and 11, exemplaryalignment monitor 120 (see FIG. 2 and supra for more details) is formedin wafer 100 and a first interconnect level (not shown) is formed overwafer 100 and alignment monitor 120. In the example of FIGS. 9, 10 and11, the first wiring level (not shown) is used to form and electricallyconnect the photodiodes and LEDs of alignment monitor 120. The firstinterconnect level is the level having polysilicon interconnects. Asecond interconnect level 165 including a dielectric layer 170 anddamascene metal contacts 175 is formed over the first interconnectlevel. A third interconnect level 180 including a dielectric layer 180and damascene metal wires 190 is formed on a top surface of secondinterconnect level 165. A fourth interconnect level 195 including adielectric layer 200 and dual damascene metal wires 205 is formed on atop surface of third interconnect level 180. A fifth interconnect level205 including a dielectric layer 210 and dual damascene metal wires 215is formed on a top surface of fourth interconnect level 195.

A damascene process is one in which wire trenches or via openings areformed in a dielectric layer, an electrical conductor of sufficientthickness to fill the trenches is formed on a top surface of thedielectric, and a chemical-mechanical-polish (CMP) process is performedto remove excess conductor and make the surface of the conductorco-planar with the surface of the dielectric layer to form damascenewires (or damascene vias). When only a trench and a wire (or a viaopening and a via) is formed the process is called single-damascene.

A dual-damascene process is one in which via openings are formed throughthe entire thickness of a dielectric layer followed by formation oftrenches part of the way through the dielectric layer in any givencross-sectional view. All via openings are intersected by integral wiretrenches above and by a wire trench below, but not all trenches needintersect a via opening. An electrical conductor of sufficient thicknessto fill the trenches and via opening is formed on a top surface of thedielectric and a CMP process is performed to make the surface of theconductor in the trench co-planar with the surface the dielectric layerto form dual-damascene wires and dual-damascene wires having integraldual-damascene vias.

A region 220 of interconnect levels 165, 180, 195 and 205 contains nowires or contacts so as to not block incident light Xnm striking thephotodiode of monitor 120 as well as to not block emitted light Ynm fromthe LED of alignment monitor 120. Emitted light Ynm is not highlydirectional and spreads over a wide arc as it is emitted. In one exampleincident light Xnm is incident at an angle of about 90° to the topsurface of wafer 100. In one example, emitted light Ynm is emitted at anangle within about 15° from the incident angle of incident light Xnm.

As illustrated in FIGS. 9, 10 and 11, wafer 100 is ready for fabricationof a next BEOL fabrication level above fourth interconnect level 205. Inone example, this would involve forming a new dielectric layer on a topsurface of dielectric layer 210, forming a photoresist layer on the topsurface of the new dielectric layer and patterning the photoresist layerin a photolithography tool as illustrated any of FIG. 12, 13, 14, 15 or16 and described infra.

In FIG. 9, a top surface of fourth dielectric layer 210 is planar andparallel to a top surface of wafer 100. In FIG. 10, the top surface offourth dielectric layer 210 in region 220 is dished downward towardwafer 100 in region 220. In FIG. 11, an opening 225 is formed ininterconnect levels 165, 180, 195 and 205 over alignment monitor 120 soas to not attenuate incident light Xnm or emitted light Ynm. It isbelieved at the current state of the art of photodiodes and LEDs that aphotodiode area of about 900 square microns (e.g., about 30 microns by30 microns) will generate about 1 microampere of current which wouldcause the LED to emit at a high enough intensity to be easily detected.

It should be understood that the interconnect levels used toelectrically connect the photodiodes and the LEDs of the alignmentmonitors may be formed in any level below that the monitors are intendedto monitor alignment for and regions 220 of FIGS. 9 and 10 and opening225 of FIG. 11 correspondingly adjusted.

FIG. 12 is schematic diagram of a first optical photolithography systemaccording to embodiments of the present invention. In FIG. 12, aphotolithography system 240 includes a X-Y-θ stage 245, a systemcontroller 250, a light source 255, a lens 260, a temperature controller265, X-alignment photo detectors (e.g., photodiodes) 270A, 270B and 270Cand Y-alignment photo detectors 275A, 275B and 275C on a mountingbracket 280, and an air temperature and flow control unit 285 havingmeans 295 for directing filtered air (or other filtered gas) at adetermined temperature onto photomask 195 at a determined flow rate, anda opening adjustable and/or moveable slit 300. Photolithography system240 also includes means (not shown) for holding photomask 145 and means(not shown) for aligning alignment targets on substrate 100 to alignmentmarks on photomask 145. Light from light source 255 passes through clearregions in photomask 145, slit 300 and lens 260 onto a layer ofphotoresist (not shown) on wafer 100. Light source 255 emits light of awavelength that causes photochemical reaction in the photoresist layer.X-alignment photo detectors 270A, 270B and 270C detect light at thewavelengths emitted by X-alignment monitors 120XA, 120XB and 120XCrespectfully and Y-alignment photo detectors 275A, 275B and 275C detectlight emitted by Y-alignment monitors 120YA, 120YB and 120YCrespectfully. In one example, X-alignment photo detectors 270A, 270B and270C and Y-alignment photo detectors 275A, 275B and 275C includephotodiodes each having a different bandpass filter to limit the rangeof wavelengths impinging on each photo detector to a respective range ofwavelengths emitted by a corresponding X-alignment monitors 120XA, 120XBand 120XC or Y-alignment monitors 120YA, 120YB and 120YC.

In operation, stage 245 steps wafer 100 under lenses 270 with the slitclosed. The slit opening is opened and light to expose just thephotodiodes of X-alignment monitors 120XA, 120XB and 120XC and thephotodiodes of Y-alignment monitors 120YA, 120YB and 120YC to light fromlight source 255. Based on the intensity of the signals from photodetectors 270A, 270B, 270C, 275A, 275B and 275C, temperature controller265 directs air temperature and flow control unit 280 to blow filteredair (or other filtered gas) at a determined temperature and flow rateover photomask 195 until signals from photo detectors 270A, 270B, 270C,275A, 275B and 275C reach predetermined values. At this point thephotomask is in thermal equilibrium. Depending upon the location ofX-alignment monitors 120XA, 120XB and 120XC and Y-alignment monitors120YA, 120YB and 120YC relative to each other and active regions of theintegrated circuit chip 105 (see FIG. 2), slit 300 may comprise twoindependently controlled slits, a first slit for exposing X-alignmentmonitors 120XA, 120XB and 120XC to light from light source 255 and asecond slit for exposing Y-alignment monitors 120YA, 120YB and 120YC tolight from light source 255. After the signal received from photodetectors 270A, 270B, 270C, 275A, 275B and 275C indicate a desired levelof mask to wafer alignment has been achieved, and mask 145 aligned towafer 100 using conventional photomask to wafer alignment means underthe control of system controller 250, slit 300 is adjusted as needed fornormal exposures of the integrated circuit chip.

Photolithography system 240 may be a step and expose system or a stepand scan system. In a step and expose system, stage 245 moves wafer 100under photomask 145, slit 300 opened to expose a full integrated circuitchip (or multiple chips and after exposure is complete the stage movesthe wafer to a new location and the process repeats. In a step and scansystem, after stage 245 moves the wafer under photomask 145 slit 300 isopened to a size less than the full size of the integrated circuit chips(or chips) and slit 300 is scanned across photomask 145 to expose wafer100 to less than whole portions of photomask 145 at any given instant oftime. Then stage 245 steps wafer 100 to a new location and the processrepeats. Optionally system 240 may be provided with a means 305 fordirecting air over lens 260 at a predetermined temperature andpredetermined flow rate until signals from photo detectors 270A, 270B,270C, 275A, 275B and 275C reach predetermined values. The temperature ofphotomask 145 and lens 260 may be controlled to the same temperature ordifferent temperatures.

FIG. 13 is a schematic diagram of a second optical photolithographysystem according to embodiments of the present invention. In FIG. 13 animmersion photolithography system 310 is similar to photolithography too240 of FIG. 12, except an immersion head 315 contains lens 260 and animmersion fluid (e.g., water) fills the space between the lens and thetop surface of wafer 100 and a fluid temperature and flow control unit320 for control of the immersion fluid temperature (and thus lens 260temperature) is provided. Immersion photolithography system may be astep and expose or a step and scan system. The temperature of photomask145 and lens 260 may be controlled to the same temperature or differenttemperatures.

FIG. 14 illustrates a first option that may be applied to the first andsecond optical photolithography systems. The first option will bedescribed using photolithography system 240 of FIG. 12 as an example. InFIG. 14, a photolithography system 325 is similar to photolithographysystem 240 of FIG. 12 except an additional light source 330 is suppliedand positioned to direct light from light source 330 through mask 145,slit 300 and lens 260 onto the photodiodes of X-alignment monitors120XA, 120XB and 120XC and the photodiodes of Y-alignment monitors120YA, 120YB and 120YC. Light source 330 may be a separate light sourcefrom 255 or combined within light source 255. With separate lightsources an optical system is provided to direct light from either lightsource 255 or light source 330 onto the optical path indicated by thedashed lines. The light from light source 330 will not causephotochemical reactions in the photoresist layer (not shown) while thelight from light source 255 will. Photolithography system 325 may be astep and expose system or a step and scan system. While the wavelengthof light produced by light source 330 is a wavelength that thephotoresist applied to wafer 100 is not sensitive to (i.e., it isnon-actinic radiation relative to the photoresist), the wavelength isone that photodiodes of X-alignment monitors 120XA, 120XB and 120XC andY-alignment monitors 120YA, 120YB and 120YC will absorb and convert tocurrent flow.

In operation, stage 245 steps wafer 100 under lenses 270 and light fromlight source 330 is directed to the photodiodes of X-alignment monitors120XA, 120XB and 120XC and Y-alignment monitors 120YA, 120YB and 120YC.Based on the intensity of the signals from photo detectors 270A, 270B,270C, 275A, 275B and 275C, temperature controller 265 directs airtemperature and flow control unit 280 to blow filtered air (or otherfiltered gas) at a determined temperature and flow rate over photomask195 until signals from photo detectors 270A, 270B, 270C, 275A, 275B and275C reach predetermined values. At this point the photomask is inthermal equilibrium, and normal photoresist exposure as described suprain reference to photolithography system 240 of FIG. 12 is performed.

FIG. 15 illustrates a second option that may be applied to the first andsecond optical photolithography systems. The second option will bedescribed using photolithography system 240 of FIG. 12 as an example. InFIG. 15, a photolithography system 335 is similar to photolithographysystem 240 of FIG. 12 except X-alignment photo detectors 270A, 270B and270C and Y-alignment photo detectors 275A, 275B and 275C on a mountingbracket 280 of FIG. 12 have been replaced with optical assembly 340connected to a spectrophotometer 345 by an optical cable 350.Spectrophotometer 345 is configured to analyze the intensity of variouswavelength bands within the signal transmitted from optical assembly345. In one example, optical assembly 345 is a lens. Operation ofphotolithography system 335 is similar to that of photolithographysystem 240 of FIG. 12. An exemplary spectrophotometer is described inU.S. Pat. No. 5,305,233 to Kawagoe et al., issued Apr. 19, 1994, whichis hereby incorporated by reference in its entity.

FIG. 16 illustrates that both the first and second option may be appliedto the first and second optical photolithography systems. The secondoption will be described using photolithography system 335 of FIG. 15 asan example. In FIG. 16, a photolithography system 355 is similar tophotolithography system 335 of FIG. 15 except additional light source330 is supplied. Photolithography system 355 may be a step and exposesystem or a step and scan system. The operation of photolithographysystem 355 is similar to that of photolithography system 325 of FIG. 14.

In FIGS. 12, 13, 14, 15 and 16, in one example, photomask 145 iscomprised of SiO₂ (not transmissive below 193 nm, coefficient of thermalexpansion of 0.5 ppm PC), SiFO₂ (not transmissive below 157 nm) or CaF₂(transmissive below 157 nm, coefficient of thermal expansion of 14 ppmPC). In one example, lens 260 and the wafer of photomask 145 arecomprised of the same material (e.g., both are SiO₂, SiFO₂ or CaF₂).While useful when the photomask and/or lens comprise SiO₂, theembodiments of the present invention are of particular usefulness whenthe lens and/or photomask comprise materials having high (e.g., greaterthan about 0.5 ppm/° C.) coefficients of thermal expansion as the amountof expansion of the mask and/or lens can be controlled to same valueregardless of room ambient temperature.

Wafers are coated with photoresist prior to being placed in the exposuresystem. After the photomask or photomask and lens temperatures areadjusted as described supra, the photomask and wafer are aligned and thephoto resist is exposed to actinic radiation through a patternedphotomask and the latent image produced developed to define a pattern inthe photoresist corresponding to a fabrication level of an integratedcircuit chip. Then etching/or ion implanting the wafer is performedfollowed by removal of the patterned photoresist layer.

Thus the present invention provides a method of monitoring andcontrolling photomask to wafer alignments compatible with sub-193 nmphotolithography (e.g., 157 nm and lower). However, the embodiments ofthe present invention may be used with wavelengths of 193 or lower.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.For example, while the present invention is directed to sub-193 nmphotolithography, the invention may be practiced with supra-193 nmphotolithography. Additionally the embodiments of the present inventionmay be practiced on substrates having a different geometry than wafers,such as rectangular substrates or wafers comprising other semiconductormaterials such as germanium, sapphire and gallium. Therefore, it isintended that the following claims cover all such modifications andchanges as fall within the true spirit and scope of the invention.

1. An apparatus an X-Y-θ stage configured to hold a semiconductorsubstrate; a light source; a lens; a mask holder configured to hold aphotomask between said light source and lens; a slit between said maskholder and said lens; means for aligning a set of alignment targets onsaid semiconductor substrate to a corresponding set of alignment markson said photomask, said alignment target of said set of alignmenttargets comprising at least one light emitting diode and a correspondingarray of photodiodes; means for directing incident light onto said setof alignment targets; means for measuring intensities of light, emittedfrom said one or more light emitting diodes; and a sub-system configuredto direct temperature controlled gas over said photomask based onsignals received from said means for measuring intensities of light. 2.The apparatus of claim 1, further including: a fluid filled immersionhead between said lens and a top surface of said substrate.
 3. Theapparatus of claim 2, further including means for adjusting atemperature of said fluid based on said signals received from said meansfor measuring intensities of light.
 4. The apparatus of claim 1, whereineach light emitting diode of each alignment mark of said set ofalignment marks emits light of a different wavelength; and wherein saidmeans for measuring intensities of light comprises an array of photodetectors, each photo detector of said array of photo detectorsconfigured to measure a different range of wavelengths.
 5. The apparatusof claim 1, wherein each light emitting diode of each alignment mark ofsaid set of alignment marks emits light of a different wavelength; andwherein said means for measuring intensities of light comprises aspectrophotometer configured to measure light intensities in differentwavelength ranges.
 6. The apparatus of claim 1, wherein said means formeasuring intensities of light comprises an array of photo detectors. 7.The apparatus of claim 1, wherein said means for measuring intensitiesof light comprises a spectrophotometer.
 8. An apparatus, comprising: anX-Y-θ stage configured to hold a semiconductor substrate; a lightsource; a lens; a mask holder configured to hold a photomask betweensaid light source and lens; a slit between said mask holder and saidlens; means for aligning a set of alignment targets on saidsemiconductor substrate to a corresponding set of alignment marks onsaid photomask, said alignment target of said set of alignment targetscomprising at least one light emitting diode and a corresponding arrayof photodiodes; means for directing incident light onto said set ofalignment targets; means for measuring intensities of light, emittedfrom said one or more light emitting diodes; and a sub-system configuredto direct temperature controlled gas over said lens based on saidsignals received from said means for measuring intensities of light. 9.The apparatus of claim 8, further including: a fluid filled immersionhead between said lens and a top surface of said substrate.
 10. Theapparatus of claim 9, further including means for adjusting atemperature of said fluid based on said signals received from said meansfor measuring intensities of light.
 11. The apparatus of claim 8,wherein each light emitting diode of each alignment mark of said set ofalignment marks emits light of a different wavelength; and wherein saidmeans for measuring intensities of light comprises an array of photodetectors, each photo detector of said array of photo detectorsconfigured to measure a different range of wavelengths.
 12. Theapparatus of claim 8, wherein each light emitting diode of eachalignment mark of said set of alignment marks emits light of a differentwavelength; and wherein said means for measuring intensities of lightcomprises a spectrophotometer configured to measure light intensities indifferent wavelength ranges.
 13. The apparatus of claim 8, wherein saidmeans for measuring intensities of light comprises an array of photodetectors.
 14. The apparatus of claim 8, wherein said means formeasuring intensities of light comprises a spectrophotometer.
 15. Anapparatus, comprising: an X-Y-θ stage configured to hold a semiconductorsubstrate; a light source; a lens; a mask holder configured to hold aphotomask between said light source and lens; a slit between said maskholder and said lens; means for aligning a set of alignment targets onsaid semiconductor substrate to a corresponding set of alignment markson said photomask, said alignment target of said set of alignmenttargets comprising at least one light emitting diode and a correspondingarray of photodiodes; means for directing incident light onto said setof alignment targets; means for measuring intensities of light, emittedfrom said one or more light emitting diodes; and a sub-system configuredto direct temperature controlled gas over both said photomask and saidlens based on said signals received from said means for measuringintensities of light.
 16. The apparatus of claim 15, further including:a fluid filled immersion head between said lens and a top surface ofsaid substrate.
 17. The apparatus of claim 16, further including meansfor adjusting a temperature of said fluid based on said signals receivedfrom said means for measuring intensities of light.
 18. The apparatus ofclaim 15, wherein each light emitting diode of each alignment mark ofsaid set of alignment marks emits light of a different wavelength; andwherein said means for measuring intensities of light comprises an arrayof photo detectors, each photo detector of said array of photo detectorsconfigured to measure a different range of wavelengths.
 19. Theapparatus of claim 15, wherein each light emitting diode of eachalignment mark of said set of alignment marks emits light of a differentwavelength; and wherein said means for measuring intensities of lightcomprises a spectrophotometer configured to measure light intensities indifferent wavelength ranges.
 20. The apparatus of claim 15, wherein saidmeans for measuring intensities of light comprises an array of photodetectors.
 21. The apparatus of claim 15, wherein said means formeasuring intensities of light comprises a spectrophotometer.